Implementing configurable error status code

Hi Bilal. This pull requests adds a configuration option `ErrorStatusCode`. When this option is set to 500, for example, any connection errors that happen get a 500 status code set on the response. This allows us to use IIS's error handling to deal with showing friendly errors, etc.

Some of the implementation is a little hacky. For instance, I could not figure how to craft a header packet to set the status code from within BonCodeAJP13ServerConnection.cs. Instead, I just added an HTML comment body chunk that the CallHandler.cs can detect and more easily set the status code.

It would be great if either this could be pulled in or something similar implemented.

Thanks for reading

Dominic